A water absorbent resin (SAP/Super Absorbent Polymer) is a water-swellable, water-insoluble polymeric gellant, and is mainly used for disposable articles as water absorbent articles such as a disposable diapers and a sanitary napkin, as well as an agricultural and horticultural water retaining agent, an industrial water stopping agent and the like. As such a water absorbent resin, many monomers and hydrophilic polymers have been proposed as raw materials. Among them, a polyacrylic acid (salt)-type water absorbent resin, which is formed by using an acrylic acid and/or a salt thereof as a monomer, is widely used in industrial purposes due to its high water absorption performance.
Such a water absorbent resin is produced through a polymerization step, a drying step (if necessary, removal of undried materials), a pulverization step, a classification step, a surface crosslinking step, and the like (Patent Literatures 1 to 10). Along with a performance improvement of a disposable diaper, which is a main application of a water absorbent resin, a water absorbent resin is also required to have many functions (physical properties). Specifically, many physical properties not only limited to high water absorption capacity but also including gel strength, extractable polymer content, water absorption rate, water absorption capacity under load, liquid permeability, particle size distribution, urine resistance, antibacterial physical properties, impact resistance (damage resistance), powder fluidity, deodorant property, coloration resistance (degree of whiteness), low dust property and the like, are required for the water absorbent resin. Therefore, numerous suggestions have been made on the surface crosslinking technology, additives, modification of production step, and the like.
Among the physical properties described above, with regard to liquid permeability, in accordance with increase of using amount of water absorbent resin in a disposable diaper (for example, 50% by weight or more), the liquid permeability is regarded as a more important factor in recent years. Furthermore, there have been suggested many methods for improvement or technologies for amelioration of liquid permeability under a load or liquid permeability without load, such as SFC (Saline Flow Conductivity; Patent Literature 11) and GBP (Gel Bed Permeability; Patent Literatures 12 to 14).
Furthermore, in addition to liquid permeability, water absorption rate is also an important fundamental property of a water absorbent resin, and as a method of increasing such water absorption rate, a technology of enhancing the water absorption rate by increasing the specific surface area is known. Specifically, a technology of finely controlling the particle size (Patent Literatures 15 and 16), a technology of agglomerating fine particles that have a large surface area (Patent Literatures 17 to 19), a technology of freeze-drying a hydrogel to make the product porous (Patent Literature 20), a technology of simultaneously agglomerating and surface crosslinking particles (Patent Literatures 21 to 23), a technology for performing foaming polymerization (Patent Literatures 24 to 42), a technology of foaming and crosslinking after polymerization (Patent Literature 43), a technology of incorporating gas bubbles into an aqueous monomer solution by using a microbubble generating apparatus (Patent Literature 44), a technology of using hollow fine particles in a hydrogel (Patent Literature 45), and the like have been proposed.
Known examples of the technologies for performing foaming polymerization (Patent Literatures 24 to 42) include, as a foaming agent that is used for the monomer, a technology of using a carbonate (Patent Literatures 24 to 32), a technology of using an organic solvent (Patent Literatures 33 and 34), a technology of using an inert gas (Patent Literatures 35 to 37), a technology of using an azo compound (Patent Literatures 38 and 39), a technology of using an insoluble inorganic powder or water-insoluble particles (Patent Literatures 40 and 41), a technology of polymerizing, without stirring, a slurry in which fine precipitates of an acrylic acid sodium salt at a concentration of 45 to 60% by weight are dispersed and an inert gas in the form of microbubbles is incorporated (Patent Literature 42) and the like.
In the conventional technologies for enhancing the water absorption rate by increasing the specific surface area (Patent Literatures 15 to 45 etc.), an increase in the water absorption rate to a certain extent may be expected, but the technologies do not still exhibit sufficient effects and necessitate special apparatuses or highly expensive raw materials (a large amount of surfactants or foaming agents). Furthermore, the relevant technologies have a problem that other physical properties such as liquid permeability (Patent Literatures 11 to 14 and 56 to 59), impact resistance (Patent Literature 46), bulk density (Patent Literatures 47 and 48), water absorption capacity under load (Patent Literatures 52 to 55) deteriorate and the like. That is, in general, since the water absorption rate and the specific surface area are positively correlated, while liquid permeability and the specific surface area are negatively correlated, it has been very difficult to achieve a good balance between water absorption rate and liquid permeability, which both depend large on the surface area.
Furthermore, in addition to the water absorption rate, many parameter physical properties such as the water absorption capacity under load (Patent Literatures 52 to 55) and liquid permeability (Patent Literatures 56 to 59) have been suggested. However, generally, when the water absorption rate is increased, the water absorption capacity under load (for example, AAP) or liquid permeability (for example, SFC or GBP) tends to decrease, and therefore, it is difficult to achieve a balance between them.
Also, by making the particle size smaller or foaming, the surface area increases, and the water absorption rate can be enhanced. However, increasing the surface area inevitably causes a significant decrease in the liquid permeability or the bulk density. A decrease in the bulk density makes the water absorbent resin bulky and decreases the particle strength. Therefore, a decrease in the bulk density is not preferable from the viewpoints of the transport or storage of the water absorbent resin, the use of the water absorbent resin in disposable diapers and the like.
Furthermore, the technology of dispersing gas bubbles by using a large amount of surfactants (Patent Literatures 35 and 36) has a problem that the technology causes not only an increase in the cost due to the surfactants, but also a decrease in the surface tension of the water absorbent resin, so that the amount of return after liquid once absorbed in disposable diapers and the like increases.
Also, in many of the applications of water absorbent resins, a water absorbent resin is used in hygiene products such as disposable diapers and sanitary napkins, and is compositized with white pulp. Accordingly, it is requested that the water absorbent resin be white in color from the viewpoint of the sense of cleanness, and many technologies for improving coloration have been proposed (Patent Literatures 49 to 51). However, from the viewpoints of the cost or safety of the coloration preventing agent, complicatedness of the steps, and the effect, those technologies still cannot be said to be satisfactory.